It is pretty sure that bats do not get in any way uncomfortable or confused when hanging upside down, because that is the way they live, when not flying, swimming, crawling on the ground and so on. Any animal that could not deal comfortably with its way of life would not last long, and bats have been with us since long before we were with bats.

As for why, first ask why anyone should feel dizzy from hanging upside down. Humans for one thing are not used to it, so it is natural to find things confising and dizzying when hanging by our feet. However, that is something one can easily get used to. There is another reason, and it is a good reason not  to experiment too much with practising hanging upside down.

If you take a long, thin-walled, plastic tube or a very long balloon, and nearly fill it with water, then lay it out on a level surface, it will contain the water quite comfortably. But if you hang it up by one end, the bottom end will bulge more than any other part, and if the tube is long enough, it will burst, even if you support the bottom.  If it is a proper, evenly-made tube, the bottom is where  it will burst. The reason is that all the water pushes down, so halfway down the tube only the top water is pressing on it, but at the bottom all the water is pressing on it. So near the bottom is where it is likeliest to break.

Now, in one way bats and humans are like that ballon or tube full of liquid: our blood vessels are like long, thin bags of blood. They are living, tough, bags of blood, but bags all the same, and the finest blood vessels have very thin walls. Usually we walk with our heads up, so the pressure in the blood vessels in our heads is low, and the vessels do not have to be very thick.  This is good, because thick vessels take a lot of room, and our brains are very crowded places.

At the same time, if the blood pressure in the brain is too low, then the brain does not get enough blood and the oxygen and foods  that blood carries, so we get dizzy and are likely to faint. To manage that, the muscles in the walls of the blood vessels (and a lot of other muscles as well)  squeeze on the blood vessels lower down, to get the right amount of blood up to the brain. If you stand on your head blood flows down, and the muscles soon stop pressing, because if they did not, the blood vessels in your brain would burst. But, having stopped pressing, they cannot change what they are doing all at once, so if you suddenly stand up again, the blood flows out of your head, and you feel dizzy and might even faint.

Beware of experimenting with that sort of thing; it can lead to burst blood vessels in the brain, and that is a very serious matter.

But why us, and not bats, you ask?

Well, you are probably between one and two metres tall, enough to build up a dangerously high pressure in your blood vessels. Bats are just a few centimetres from tail to nose. For them the difference in pressure is hardly noticeable, no matter which way up they may be.

I hope that is what you wanted to know!

Jon

Dizziness is a sensation humans describe when they feel a sense of motion, even when not moving. It can be associated with queasiness or nausea, and sometimes vomiting. Other types of dizziness include motion sickness and vertigo, which often manifests itself as a spinning feeling, or other sensations such as light-headedness or heavy-headedness.

It is impossible to know for sure whether or not an animal is dizzy because it cannot communicate such feelings. However, it is possible to infer an animal is dizzy from how it behaves. For example, if an animal is aimlessly walking in circles, it is probably dizzy.

Motion sickness occurs when there is excessive stimulation of the inner ear or from a conflict between sensory information from different sources, such as from the inner ear and the eyes. The balance mechanism of the inner ear is complicated, and includes sensors that detect both movement and orientation with respect to gravity, even when an individual is not moving. Bats have such a balance mechanism, and in addition use echolocation.

The parts of the inner ear that are important for orientation with respect to gravity are called the otolith organs: the utricle and the saccule. It is these parts of the inner ear that would be activated while the bat was hanging upside down. Stimulating these parts of the inner ear, however, would not necessarily lead to dizziness, especially in a dark cave where there is no conflict between information from the inner-ear balance mechanism and vision.

The bottom line is that bats are used to hanging upside down without showing any behavioural changes that would suggest dizziness or motion sickness. But because we cannot ask a bat directly whether or not it is dizzy, we can't be certain about the effects of hanging upside down.

Joe Furman, Editor of the Journal of Vestibular Research, University of Pittsburgh, Pennsylvania, US

When you think of bats, you usually think of them in one of two conditions: hanging upside down resting, or flitting about pulling high-g turns in the dark. So why don't they get dizzy?

Bats have evolved a number of adaptations to allow them to hunt and hang without the problems that humans would face.

First, some bats have specialisations in the vestibular portion of their inner ears - the portion that generates sensory signals for controlling balance. Their sacculus, which in humans acts as a gravity sensor to help us stand upright, is slightly rotated forwards. This lets it act more as a pitch detector, which is more useful in flight.

Second, their semicircular canals, which sense rotation of the head, have an internal structure more like a bird's than a human's. This probably allows them to make high-speed turns without the fluid in the canals sloshing back and forth too much.

Lastly, if you photograph bats in flight with a high-speed camera, you notice that they keep their heads very stable except in the most violent turns.

But it is how bats sense the world that probably gives them immunity to dizziness. All the vestibular system does is tell you about changes in acceleration of your head. It requires other senses to pin down your position and motion in the outside world. We primarily use vision to do this, but vision is very slow. Anything you look at that takes a second or less to cross 30 degrees of your vision appears smeared. Echolocating bats, while not blind, rely more on biosonar, an especially precise form of hearing that lets them build up 3D images from echoes.

Echolocating bats emit brief sonar chirps from 30 to more than 150 times per second, and respond to changes in echoes of less than a microsecond. These bats integrate echolocation with their vestibular system, so they are working with a faster, more precise positioning system than humans do with vision. Because dizziness and motion sickness usually arise when signals from the vestibular system conflict with those from other sensing systems, bats are less likely to show motion sickness than other mammals.

Seth Horowitz, Assistant professor, neouroscience, Brown University, Rhode Island, US

Professor Horowitz’ reply reminds me of a holiday in the Magaliesberg where hollows in the walls of the chalets turned out to be housing colonies of very small, very cute rufous bats, some sort of pipistrelle, I should think. They flew all over, including into the light in the chalet. One settled on the curtain and as we crowded around it looked at us and opened its mouth. We realised that as it inspected us visually, it also was doing the same thing by echolocation. No doubt it was mentally correlating the two images, but of course we were in no position to establish that definitely.

One of those marvellous, warm memories for a biologist.

I am slightly puzzled by the emphasis that some respondents place on the vestibular apparatus. After all, perching bats don't spend all their time whirling about. And I have never noticed people becoming particularly dizzy just because they are head down.

Of course, in a fairground ride or aerobatic plane, where one might well be frequently upside down, one may get dizzy but the most violent spell of dizziness I ever have experienced, so severe as to induce something of a state of shock, was perfectly erect in a fairground "wall of death", where I experimented by moving my head towards and away from the axis of spin. Hanging upside down as such, has never caused me the slightest dizziness. Am I unusual in this? On getting up again however, one has rushes of blood back into the body, which of course can induce faintness, but that is hardly the same thing surely?

Cheers,

Jon